Electrodeposition has long been used as a method for plating objects with a particular material. Electrodeposition is a relatively easy way to coat an object with a material, such as copper, where it would be difficult to provide a thin uniform coating by other methods.
Electrodeposition has been used to plate gravure cylinders, which are commonly used in printing processes. A gravure cylinder is plated with a thin layer of a material such as copper and then the desired print is etched into the copper layer. In such cases, a steel or aluminum cylinder usually forms the substrate which supports the electrodeposition layer. Once the print run is finished, the gravure cylinder must be reconditioned so that a different print may be etched into the cylinder. Reconditioning requires that the electrodeposition layer be removed, at least partially, to remove the previous etching so that a new electrodeposition layer may be placed on the cylinder. As before, once the new layer of electrodeposition material covers the cylinder, the desired etching may be made for future printing.
An electrodeposition system requires an ionic fluid bath which contacts the object to be plated. The ionic fluid bath includes ions of the deposition material. A supply of the electrodeposition material must also be in contact with the ionic fluid bath to supply the fluid bath with additional ions once the plating process begins.
For example, when a gravure cylinder is to be plated with copper, the cylinder will be submerged or rotated while in contact with a fluid bath containing copper ions. A tray of copper nuggets or copper bars would typically be submerged in the fluid bath in proximity to the gravure cylinder. An electrical field would then be established across the object and the supply of deposition material. The charge applied to the object would be opposite to the charge of the ions in the fluid bath thus attracting the ions to the object. In this manner, the ions are deposited on the object forming a layer or plating on the object. Meanwhile, additional ions break free from the deposition material supply and enter the fluid bath generally replacing the ions that were attracted to the object. In the gravure cylinder . example, the cylinder can be rotated through the fluid bath while the electrodeposition process takes place so that a layer of deposition material will be applied generally over the entire surface of the gravure cylinder.
Often, during the electrodeposition process, oxides and other contaminants are given off when the electrodeposition material supply ionizes; that is, when ions break face from the deposition material supply and enters the fluid bath. This is due largely to impurities which exist in the supply material. Thus, the ionic fluid which is used in the fluid bath is usually cycled through a larger reservoir of ionic fluid. Before returning to the fluid bath, the ionic fluid is filtered and resupplied to the fluid bath. In Bergin et al., U.S. Pat. No. 3,923,610, a method is disclosed for copper plating a gravure cylinder in which a typical plating system is used. A cylinder is rotatably mounted in contact with an electrolyte which is retained in a vat. The electrolyte consists essentially of a solution of copper sulfate and sulfuric acid in water. The cylinder is partially immersed in the ionic fluid bath and rotated while an electric field is established across the cylinder and a solid copper supply.
A problem with prior art devices such as the Bergin device is that those devices were not able to deposit material on the object being plated in a precise uniform manner. This presented problems when plating or reconditioning objects such as gravure printing cylinders, which require an extremely precise and 15 uniformly smooth surface. To obtain such a desired uniform surface using prior art plating devices, an old layer of deposition material was first removed and then the object was thoroughly cleaned. Following its cleaning, the object was typically plated with a relatively thick layer of the electrodeposition material and then the layer underwent a refining process which involved rough cutting the electrodeposition layer to a generally uniform finish, fine cutting the rough cut finish, and then polishing the surface until it had the desired smooth and uniform characteristics. This process, however, was time consuming and wasted substantial electrodeposition material.
The inventor has determined that the nonuniformity of the electrodeposition layer achieved using prior art plating apparatus is caused largely by uneven dispersion of the ions as they are attracted to the object and by contaminants which enter the fluid bath and become attached to the object which is being plated. It would be advantageous to prevent contaminants from being introduced to the fluid bath from either the electrodeposition material supply or from the recycled ionic fluid which is introduced into the fluid bath. Additionally, it would be advantageous to uniformly disperse of ions in the fluid bath as ions are introduced into the fluid bath from either the electrodeposition material supply or from the ionic fluid being recycled to the fluid bath. For example, a problem with the prior art systems was that ionic fluid was introduced into the fluid bath through orifices which generally sprayed columns of fluid into the fluid bath. Such columnar spraying was found to cause uneven plating of the cylinder leaving high spots and low spots in the material deposited on the cylinder according to the location of the supply orifices.
Other methods and devices for electrodeposition of a material are disclosed in patents such as Datwyler, U.S. Pat. No. 4,437,942 and Katano et al., U.S. Pat. No. 4,405,709. However, nothing in these prior art references adequately addresses the problems discussed above.